1. Field of the Invention
The present invention is in the field of optics, specifically in optical couplers for bi-directionally coupling optical radiation between two waveguides. It is also in the field of mode transformer between two waveguides.
2. Prior Art
The mode coupler between two different waveguides is an essential part of an optical system where the lightwave (mode) from one optical component is coupled into another component. In optical communication, the mode coupler between an optical fiber waveguide and a high index difference (difference in the refractive indices of core and cladding) planar waveguide is crucial for successful implementation of integrated optics in fiber communication. Therefore, developing an efficient mode coupling between two waveguides has been the subject of intense research.
When coupling the modes between two waveguides with different index differences and/or core indices, high coupling loss arises due to the difference in the mode size, shape, and mode velocity. For example, the index difference, and the mode of a fiber optic waveguide are different from those of a high index difference planar waveguide, resulting in a high coupling loss when the fiber optic waveguide and the high index difference planar waveguide are coupled directly. The index difference of a fiber is smaller than that of high index difference waveguides, making the fiber mode larger than the waveguide mode. In addition, the core index of the fiber optic waveguide is lower than that of the high index difference planar waveguide causing a mode velocity difference between two waveguides. When such a change in mode properties takes place too quickly, high power loss arises.
There have been several approaches to achieve efficient mode coupling between two waveguides with different index difference, including mode transformation by tapering the dimension of high index difference waveguide. Mode transformation by a taper has been shown in various publications. Over the tapering region of the high index difference waveguide, the thickness of the waveguide core is gradually tapered down from that of the normal guiding region to 0 thickness. As the mode travels from the normal guiding region of the high index difference waveguide into the tapering region, the mode experiences decreasing amount of the core material. The fraction of the mode field distribution that exists outside the core material increases, changing the mode size. The index of the waveguide that the mode experiences is effectively changed by the presence of the taper. In other words, the xe2x80x9ceffective indexxe2x80x9d is gradually changed by the taper. By gradually changing the effective index from that of the low index difference waveguide to that of the high index difference waveguide, the mode coupling can be achieved between two waveguides without high loss.
Mode transformation based on tapering is shown in the prior art including IEEE Photonics Technology Letters, Vol. 5, No.9, September 1993 by Brenner et al. In this publication, the core of the planar waveguide is vertically tapered down from that of the regular waveguide. The mode size propagating in the tapered region increases due to the reduction of the effective index, and thus the reduction of the effective index difference. This publication shows the gradual mode transformation occurring in one waveguide due to the presence of a taper.
U.S. Pat. No. 5,199,092, issued to Stegmueller et al. shows the coupling of modes between two different waveguides: one broad and one narrow. The two waveguides run parallel to one another and are superimposed with each other to provide a superimposing waveguide guidance. During the superimposed waveguide guidance, one of the two waveguides is tapered down in vertical dimension, while the other waveguide dimension is kept constant. The role of the tapered waveguide is to provide a gradual effective index change, and thus mode transformation, same as the cases in journal publications including that by Brenner et al. The only difference of this technology from the technologies available in publications is the superimposition of the narrow waveguide in the broad waveguide, providing waveguiding in the broad waveguide once the narrow waveguide is completely terminated by the vertical taper. The broad waveguide is surrounding the narrow waveguide over the whole waveguiding distance. The presence of the broad waveguide helps guiding the mode once the mode transformation is complete.
In accordance with the invention, using a dual-taper, achieves low-loss mode coupling between two waveguides having different index differences, core indices, and dimensions. This apparatus can be used to couple the optical mode from an optical fiber, whose typical single-mode dimension of the core is approximately 10 xcexcm in diameter, to the mode in a high index difference planar waveguide, whose single-mode dimension of the core is less than that of an optical fiber.
It is an objective of the invention to provide a device for transforming the mode between two waveguides with different mode sizes and indices. It is another objective of the invention to provide a device to enable low-loss coupling between the optical fiber waveguide mode and the high index difference planar waveguide mode.
In the invention, the mode undergoes a low-loss transformation between a low index difference waveguide and a high index difference waveguide by traveling through the coupling region containing a dual-taper. The dual-taper provides a gradual change in the mode properties necessary for low-loss, bi-directional mode transformation. Both the low index difference and high index difference waveguides are tapered, in opposite directions. These two oppositely running tapers are placed so that there is an overlap of two waveguides, with the smaller waveguide embedded in the larger waveguide.
It is an objective of the invention to show that the dual-taper structure enhances mode transformation efficiency between two waveguides. It is another objective of the invention to demonstrate the two tapered waveguides should be overlapped, or in contact, for low-loss mode transformation.
To apply the invention for coupling the modes between an optical fiber and a high index difference waveguide, the low index difference waveguide can be chosen to have similar index difference, core index, and mode size as an optical fiber. The mode from the fiber is initially coupled to the low index difference waveguide having similar properties. Therefore the coupling is achieved with low loss due to the similarity of the modes. Once coupled, light is guided in the low index difference waveguide. Then the mode and the effective index of the low index difference waveguide are gradually changed to that of the final waveguide by the dual-taper.